In the art of magnetic resonance examination, the birdcage resonator, also known as birdcage coil, is a well-known volume radio frequency coil design for generating a radio frequency magnetic excitation field B1 to be applied to nuclei of or within a subject of interest for magnetic resonance excitation, wherein the subject of interest is positioned at least partially within the birdcage coil, which in turn is arranged within a static, homogeneous magnetic field B0 arranged substantially perpendicular to the radio frequency magnetic excitation field B1.
In the art, birdcage resonators are known to serve as radio frequency transmit coils and/or radio frequency receive coils. They are usually operated in resonance at a radio frequency corresponding to the Larmor frequency, which depends on the strength of the static magnetic field B0 and the gyromagnetic magnetic ratio of the species of nuclei under consideration.
U.S. Pat. No. 4,680,548 describes the volume radio frequency coil design, later referred to as the “birdcage coil” for its appearance, as a magnetic resonance radio frequency coil having a pair of conductive loop elements spaced along a common longitudinal axis. Each of the loop elements may include a plurality of serially-connected capacitive elements spaced along the loop peripheries. A plurality of axial conductive elements (commonly referred to as “rungs”) electrically interconnect the conductive loop elements at points between adjacent ones of the serially connected capacitive elements. In the high-pass embodiment of the radio frequency coil, the axial conductive segments may be wires, conducting tubes or flat conductive tapes whose inherent inductance is needed for proper coil operation. A band-pass embodiment of the coil is realized by including capacitive elements in each of the axial conductive segments. Birdcage coils are known to have as many resonant modes as there are radial or axial conductive segments. The preferred excitation mode for the birdcage coil is the one in which a generated radio frequency magnetic excitation field B1, when operated as a transmit coil, is as homogenous as possible. This is the case for resonant modes whose current distribution in the rungs is proportional to sin θ or cos θ, respectively, wherein θ denotes the azimuthal angle measured circumferentially about the birdcage coil axis.
U.S. Pat. No. 4,680,548 further describes to operate the birdcage coil in a quadrature excitation mode in which the birdcage coil transmits a circularly polarized radio frequency magnetic field, known to maximally interact with nuclei spins. To this end, the birdcage coil is excited at two input capacitors located at right angles relative to one another, for instance along the circumference of one of the conductive loop elements, by two radio frequency sources that are electrically 90° out of phase relative to one another. In the case of quadrature excitation, the magnitude of the currents in each rung is equal while the relative phase angle increments in a linear manner with the azimuthal angle θ.
US patent application publication US 2010/0036237 A1 describes a detector unit for arrangement in a field generating unit of a magnetic resonance device. The detector unit has an RF transmission/reception system for transmitting RF pulses into, or receiving magnetic resonance signals from, an examination volume of the field generation unit. The RF transmission/reception system surrounds a patient tunnel at a radial distance from a tunnel axis thereof, and is divided into two sub-systems located at an axial distance between 10 cm to 50 cm from each other along the direction of the tunnel axis, so as to form a substantially annular cavity or interstice therebetween. Each one of the sub-systems is fashioned as semi-birdcage resonator, each semi-birdcage resonator comprising a ferrule and a ring and a number of antenna rods that start from the respective ferrule and are connected with their ends at the ring that forms part of an RF shield.
U.S. Pat. No. 4,837,515 A describes a radio frequency coil for nuclear magnetic resonance imaging that comprises two annular conductors which are coaxially disposed at opposite ends of a prescribed axis and at least one pair of longitudinal conductor groups which are symmetrically disposed in parallel to the prescribed axis and extend longitudinally from one annular conductor to the other. Each longitudinal conductor group comprises a plurality of longitudinally extending electrical conductors whose ends are secured and electrically connected to the annular conductors. The longitudinal conductors are preferably in the form of electrically conducting wires, tubes, or plates. The annular conductors can be in the form of one-piece rings or in the form of one or more pairs of arcuate plates which are electrically connected with one another and disposed about the prescribed axis in the form of a ring. In one embodiment, one of the longitudinal conductor groups of each pair is transversely divided in two at its mid portion. The lower ends of the longitudinal conductors in the upper half of the divided longitudinal conductor group are short-circuited, and similarly the upper ends of the longitudinal conductors in the lower half of the divided longitudinal conductor group are short circuited by suitable means. The two halves are connected with one another by capacitive coupling.